With the miniaturization of electronic devices, the electronic components used therein have also become progressively smaller and of higher performance, in addition to which the components are automatically mounted on printed circuit boards during assembly of the electronic devices. Surface-mounted electronic components are housed in carrier tape with receiving pockets that are successively formed by embossing in accordance with the shapes of the electronic components. After loading the electronic components into the receiving pockets, a cover film is laid as a lid material over the top surface of the carrier tape, and the ends of the cover film are continuously heat-sealed in the longitudinal direction with a heated seal bar to form packages of electronic components. As cover film materials, those having a heat seal layer of thermoplastic resin laminated onto a substrate of biaxially stretched polyester film are used.
In recent years, there have been advances in miniaturization, weight reduction and thinning of various electronic components such as capacitors, resistors, ICs, LEDs, connectors and switching elements, such that if the difference between the maximum value and the minimum value, i.e. the “range”, of the peel strength when peeling the cover film is large, then the carrier tape can vibrate violently, causing the electronic components to spill out and resulting in mounting errors. Additionally, with sudden increases in mounting speed, the peeling rate of cover films has greatly increased to 0.1 seconds or less per tact, so that large impact stresses are applied to the cover film during peeling. As a result, when the peel strength is too high, the cover film may rupture.
As methods of reducing variations in peel strength of the cover film, a method of patterning the heat seal layer into islands and a method of mixing an incompatible resin into the intermediate layer or the heat seal layer to cause cohesive failure in the layer have been proposed (see, e.g., Patent Documents 1 and 2). However, the miniaturization of electronic components has placed higher demands to reduce variations in peel strength, so there are cases in which the required performance cannot be satisfied even by such methods.
Additionally, a method of suppressing variations in peel strength by limiting the softening temperature of the resin used in the intermediate layer (see, e.g., Patent Document 3) and a method of using a metallocene linear low-density polyethylene (LLDPE) of a particular specific weight as the intermediate layer to form a layer with a low Young's modulus as the adhesive layer between the intermediate layer and the substrate layer, thereby preventing the propagation of stresses to the substrate layer (see Patent Document 4) have been proposed. However, even when using these methods, it is difficult to adequately suppress film ruptures when peeling the cover film from the carrier tape at high speed.    Patent Document 1: JP H7-223674 A    Patent Document 2: JP H4-279466 A    Patent Document 3: WO 2004/094258 A    Patent Document 4: JP 2006-327624 A